Biology Now, 2e

A Deadly Inheritance ■ 139

syndrome, in which females receive only one X

chromosome, is an exception to this rule. Turner

syndrome individuals tend to live long, healthy

lives with mild to moderate reproductive issues.

Similarly, Down syndrome individuals receive

three copies of chromosome 21 and can live rela-

tively long lives, but they still suffer from mild

to moderate intellectual and developmental

disabilities. Changes in chromosome structure

can also have dramatic effects. For example,

cri du chat syndrome, caused by a deletion on

chromosome 5, results in slowed growth, a small

head, and developmental delays.

Looking for Loci

In his 1954 paper describing the disease, Aldrich

suggested that since primarily males inherit

the syndrome, it is caused by a mutation on a

sex chromosome. Aldrich recognized that the

family’s disease was tightly linked to gender

and concluded that the disease was caused by

Parents

Offspring

Eggs

Sperm

Meiosis

Gamete precursor cells

Boys will inherit an X

from Mom and a Y from

Dad.

Girls will inherit an X

from Mom and an X from

Dad.

XX

XX

XX

XY

XY

XY

X

X

X

X

X

X

Y

× Y

Figure 8.5

Dad’s chromosomes determine baby’s sex

Q1: What are the odds that a given egg cell will contain an X

chromosome? A Y chromosome? What are those odds for a sperm

cell?

Q2: If a couple has two daughters, does that mean their next two

children are more likely to be sons? Explain your reasoning. (Hint: Refer

back to “What Are the Odds?” on page 124.)

Q3: Sisters share the same X chromosome inherited from their father,

but they may inherit different X chromosomes from their mother. What is

the probability that brothers share the same Y chromosome? What is the

probability that brothers share the same X chromosome?

A German pediatrician who lived from 1898 to 1978,

Alfred Wiskott described the cases of three young

brothers with a serious bleeding disease—what

would later be called Wiskott-Aldrich syndrome.

Wiskott deduced that the boys had inherited a

gene from their parents that was causing the

illness.

ALFRED WISKOTT

chromosome, whereas females have two X

chromosomes. The Y chromosome in humans

is much smaller than the X chromosome.

Because human females have two copies of the

X chromosome, all the gametes (eggs) they produce

contain one X chromosome, passed on to their

offspring. Males, however, have one X chromosome

and one Y chromosome, so half of their gametes

(sperm) will contain an X chromosome and half

will contain a Y chromosome (Figure 8.5).

Each chromosome has a particular struc-

ture, with genes arranged on it in a precise

sequence. Any change in the chromosome

number or structure, compared to what is typi-

cal for a species, is considered a chromosomal

abnormality. The two most common types of

chromosomal abnormalities in humans are

changes in the overall number of chromosomes

and changes in chromosome structure, such as

a change in the length of an individual chro-

mosome (Figure 8.6).

Changes in the number of chromosomes in

humans are usually lethal, although Turner